The panel are investigating all aspects of Nasa's manned space flight programme

Top Nasa officials have expressed concern over the effect that privatising the United States space agency had, as experienced civil servants were replaced by private contractors in a bid to cut costs.

Ron Dittemore, the space shuttle programme director, and Jefferson Howell, director of the Johnson Space Center, both questioned the wisdom of the move.

But both men stressed the problem of dependence on private contractors had not compromised astronaut safety, which they said remained Nasa's priority.

Their comments were made as they gave evidence at the first day of public hearings into the space shuttle Columbia disaster.

The shuttle, the oldest in Nasa's fleet, was destroyed when it broke up on re-entry into the Earth's atmosphere on 1 February, killing its crew of seven astronauts.

Brain drain

Members of the Columbia Accident Investigation Board (CAIB), led by retired admiral Harold Gehman, quizzed the officials on a Nasa report in 2001 which noted that since 1993 the space agency had lost 50% of its civil servant technicians.

It is still not known what caused Columbia to break up

The report warned that this change represented a "significant loss of skills and experience" and "a serious threat to safety and mission
success".

Mr Howell said that, of the 10,000 people who currently work at the Johnson Space Center, just 3,000 were civil servants, while the rest were employees from private aerospace sub-contractors.

Mr Dittemore told the 13-member CAIB panel that while much of the cutting back on civil servants had been justified the trend had had a damaging effect on employee thinking.

Mr Dittemore said the space programme had started developing a "going out of business mentality" and begun "slowly losing the checks and balances and healthy tensions" necessary to
ensure safety.

Safety warning

Also giving evidence at the hearing was Henry McDonald, a former Nasa official who led a study three years ago criticising the way the space agency dealt with safety risks.

Mr McDonald, the former director of research at Nasa's Ames Research Center, said the loss of civil servant employees had "eroded" shuttle safety as the emphasis shifted to speed and cost.

He said Nasa used a flawed system to assess flight risk according to which a perception prevailed among staff that "if I've flown 20 times, the risk is less than if I've flown just once".

Admiral Gehman described Mr McDonald's report three years ago as "eerily prescient".

The CAIB board, charged with examining all aspects of Nasa's manned flight programme in order to discover what caused the shuttle to break up, will hold intermittent public hearings for as long as the investigation is underway.